Thoughts about D

[Walter Bright]

On 11/30/2017 3:51 PM, Nicholas Wilson wrote:
> On Thursday, 30 November 2017 at 18:18:41 UTC, Jonathan M Davis wrote:
>> But I have a hard time believing that the cost of assertions relates to 
>> constructing an AssertError unless the compiler is inlining a bunch of stuff 
>> at the assertion site. If that's what's happening, then it would increase the 
>> code size around assertions and potentially affect performance.
>>
>> - Jonathan M Davis
> 
> Indeed, if DMD is not marking the conditional call to _d_assert (or whatever it 
> is) 'cold' and the call itself `pragma(inline, false)` then it needs to be 
> changed to do so.

Instead of speculation, let's look at what actually happens:

---------------------------------
   void test(int i) {
     assert(i, "message");
   }
---------------------------------
dmd -c -m64 -O test
obj2asm -x test.obj
---------------------------------

__a6_746573742e64:
         db      074h,065h,073h,074h,02eh,064h,000h      ;test.d.
__a7_6d657373616765:
         db      06dh,065h,073h,073h,061h,067h,065h,000h ;message.

_D4test4testFiZv:
0000:           push    RBP
0001:           mov     RBP,RSP
0004:           sub     RSP,040h
0008:           mov     010h[RBP],ECX
000b:           cmp     dword ptr 010h[RBP],0
000f:           jne     $+3Ah
--- start of inserted assert failure code ---
0011:           mov     R8D,5                     // line number
0017:           lea     RAX,FLAT:_BSS[00h][RIP]
001e:           mov     -018h[RBP],RAX            // filename.ptr
0022:           mov     qword ptr -020h[RBP],6    // filename.length
002a:           lea     RDX,-020h[RBP]            // &filename[]
002e:           lea     RCX,FLAT:_BSS[00h][RIP]
0035:           mov     -8[RBP],RCX               // msg.ptr
0039:           mov     qword ptr -010h[RBP],7    // msg.length
0041:           lea     RCX,-010h[RBP]            // &msg[]
0045:           call      L0
--- end of inserted assert failure code ---
004a:           mov     RSP,RBP
004d:           pop     RBP
004e:           ret
-------------------------------------------

26 bytes of inserted Bloaty McBloatface code and 15 bytes of data. My proposal:

_D4test4testFiZv:
0000:           push    RBP
0001:           mov     RBP,RSP
0004:           sub     RSP,040h
0008:           mov     010h[RBP],ECX
000b:           cmp     dword ptr 010h[RBP],0
000f:           jne     $+01h
0011:           hlt                                // look ma, 1 byte!
0012:           mov     RSP,RBP
0015:           pop     RBP
0016:           ret

1 byte of inserted code, and the data strings are gone as well.